This invention relates to defibrillators and, more particularly, to converting a monophasic defibrillator to a defibrillator capable of applying a biphasic defibrillation pulse to a patient.
One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique for restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.
Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patient""s chest. The switching mechanism used in most contemporary external defibrillators is a high-energy transfer relay. A discharge control signal causes the relay to complete an electrical circuit between the storage capacitor and a wave shaping circuit whose output is connected to the electrodes attached to the patient.
The relay used in contemporary external defibrillators has traditionally allowed a monophasic waveform to be applied to the patient. A typical monophasic defibrillator is shown in FIG. 1. As illustrated in FIG. 1, a host control circuit 10 activates a capacitor-charging circuit 12 to charge a storage capacitor C1 up to a high voltage level. Once capacitor C1 is charged, the defibrillator is ready to apply a defibrillation pulse. To apply a defibrillation pulse, the host control circuit 10 activates control line XFER, which closes relay switches SW1 and SW2. Relay switches SW1 and SW2 may be mechanical relays or solid state switching devices, and in some cases may be replaced by a single relay switch, such as switch SW1. Once relay switches SW1 and SW2 are closed, a monophasic defibrillation pulse travels from the capacitor C1 to the patient 14. The path of the pulse energy is from the positive terminal of the capacitor C1 to a line 20 and through switch SW1. Next, the pulse passes through a line 30 and through the patient 14 in the direction of arrow 16. Finally, the pulse passes through a line 32, switch SW2, and another line 22 to the negative terminal of the capacitor C1.
Once the storage capacitor C1 is charged, if the operator decides not to apply a defibrillation pulse to the patient, the capacitor is then discharged by the control signal DUMP. The control signal DUMP may also be activated by a xe2x80x9ctime-outxe2x80x9d period, or when the power to the defibrillator is turned off, or by other selected events. To discharge the capacitor C1, the host control circuit 10 activates the control signal DUMP so as to close the switch SW3 and short out the remaining energy from the capacitor C1 through switch SW3 and a dump resistor R1. Dump resistor R1 limits the current from the capacitor C1 through the switch SW3 so as to prevent damage to the circuit components. by discharging capacitor C1 relatively slowly.
While contemporary external defibrillators such as those described above have traditionally applied a monophasic waveform to a patient, it has recently been discovered that there may be certain advantages to applying a biphasic rather than a monophasic waveform to the patient. For example, preliminary research indicates that a biphasic waveform may limit the resulting heart trauma associated with the defibrillation pulse.
While defibrillators applying biphasic waveforms may have certain advantages, the cost of upgrading from monophasic to biphasic defibrillators can be significant. Consumers and manufacturers who have made substantial investments in the purchase and development of conventional monophasic defibrillators are faced with the costly expense of purchasing or developing entirely new biphasic defibrillators if they wish to upgrade to biphasic technology.
The present invention is directed to providing an apparatus that overcomes the foregoing and other disadvantages. More specifically, the present invention is directed to an upgrade kit for converting a conventional monophasic defibrillator into a defibrillator that is capable of applying a high-energy, biphasic defibrillation pulse to a patient.
In accordance with this invention, a defibrillator that applies monophasic defibrillation pulses to a patient may be converted into a defibrillator capable of providing biphasic defibrillation pulses. This conversion is significantly less expensive than the purchase or development of an entirely new biphasic defibrillator and reduces the training required for those who are already familiar with the controls for operating the monophasic defibrillator.
In accordance with further aspects of this invention, the upgrade kit is easily connected to and at least partially controlled by the control circuit of the monophasic defibrillator. The upgrade kit connects to easily accessible circuit components such as the terminals of the storage capacitor. In addition, an upgrade control circuit of the upgrade kit uses some of the control signals that are commonly available in most monophasic defibrillator control circuits. More specifically, the control signals for performing functions such as activating the monophasic pulse, and for dumping unwanted stored energy, may be used. These signals are commonly available in most monophasic defibrillator control circuits and are carried by control lines that can be easily coupled to and implemented by the upgrade kit. For the defibrillation pulse control signal, the upgrade kit is able to compensate for any delay time in the relay switches of the original defibrillator by delaying the activation of the faster upgrade kit switches until the slower relay switches have had time to close.
In accordance with further aspects of this invention, the upgrade kit uses a discharge method that will allow it to apply a proper biphasic defibrillation pulse regardless of the initial energy settings of the host defibrillator. The upgrade kit accomplishes this by using two measurements that are taken near the beginning of the biphasic defibrillator pulse and using the ratio of the two measurements to determine the desired length of the biphasic pulse. The desired length of the biphasic pulse may be determined by a look-up table in the upgrade control circuit. Use of this method allows the upgrade kit to apply a proper biphasic defibrillation pulse without needing information regarding the energy level to which the storage capacitor has been charged. This eliminates the need for a serial interface between the host control circuit in regard to the selection of energy levels to which the storage capacitor may be charged.
As will be readily appreciated from the foregoing description, the upgrade kit of the present invention is easily inserted into a conventional monophasic defibrillator. By using control signals that are readily available, and by not requiring a serial interface between the host and upgrade kit control circuits, the installation of the upgrade kit is simplified. The resulting upgraded defibrillator is less costly than a new biphasic defibrillator but is capable of providing the advantageous biphasic defibrillation pulses to a patient.